This invention relates generally to cladding or coating cavities of metal objects, and more particularly to mud pump liner cavities.
Internal cavities of metal objects frequently require a cladding, or a coating, that is more corrosion, oxidation and/or wear resistant than the metal object itself. This need may arise in some cases due to high temperatures created within the cavity, exposure to a corrosive or abrasive liquid, and/or to rubbing action of an internal machine member such as a piston. An example of such a metal object is the liners in mud pumps used in oil field drilling. A mud pump is a part of the oil or gas well drilling fluid circulating system, one of five major components of a rotary drilling operation. The other components are the drill string and bit, the hoisting system, the power plant and the blowout prevention system.
Drilling fluid, usually called the "mud", in most cases consists of a mixture of water, various special chemicals including corrosion inhibitors and solid particles such as Barite to increase its density. Such fluid is continuously circulated down the inside of the drill pipe, through the bottom of the bit and back up the annular space between the drill pipe and the hole. The driving force is provided by a mud pump.
A mud pump liner is basically a heavy wall pipe section with one or two retaining rings at its outer diameter. It is the wear resistance of the inner surface that determines the liner service life. Consequently, the internal surface of the liner is desirably clad with a wear resistant material. The internal cladding layer is subjected to sliding wear by the rubber piston which can wear and cause metallic structure supporting the rubber to contact the liner cladding, thus accelerating the wear process. The cladding material is also subjected to corrosion from the drilling fluid, and metal fatigue caused by cyclic loading, especially at areas where the direction of the piston motion suddenly changes. Further, micro regions of cladding may experience sudden pressurization and depressurization. These operating conditions impose stringent metallurgical requirements on the cladding materials. An ideal cladding material should, therefore, possess high hardness and high resistance to corrosion, impact and metal fatigue. Such properties are desirably achieved by a uniform, fine grained microstructure, which has been the goal of pump liner makers of many years.
The outer, heavy wall portions of the commercially available mud pump liners typically consist of either a carbon steel, or a low alloy steel; and the liner cladding is, in most cases, a cast sleeve of iron--28% chromium alloy. The sleeve can be centrifugally cast into the steel pipe section or cast separately as a pipe, and shrink fitted into the outer pipe section, then machined to a smooth finish. These manufacturing procedures are lengthy and costly, while providing only a cast metal microstructure which is known to be chemically nonuniform, since in castings the solidification process results in natural segregation of the elemental species contained in the alloy. Furthermore, the cladding thicknesses are kept undesirably large to allow casting processes to be used. The claddings within metallic objects other than pump liners can be similarly characterized and most likely be prone to the same deficiencies.
A cladding layer made of powder metals consolidated to near 100% density and bonded to the outer steel shell appears to provide the most desirable metallurgical microstructure, due to its chemical uniformity and high ductility emanating from its fine grain size. Existing methods of application of such powder metal layers, however, are grossly inadequate in that they either produce a porous, oxide contaminated layer which is only mechanically bonded to the outer shell as in sprayed coatings, or they are superficially and only mechanically bonded to the outer shell as in brazed-on coatings. For these, and other reasons, present powder metallurgy techniques for such products have not been considered adequate.